Docs: Update expression documentation to mention no data (#61934)

Co-authored-by: Christopher Moyer <35463610+chri2547@users.noreply.github.com>

docs/sources/panels-visualizations/query-transform-data/expression-queries/index.md

@@ -219,3 +219,8 @@ For more information about expressions, refer to [About expressions]({{< relref
 
 1. Write the expression.
 1. Click **Apply**.
+
+## Special cases
+
+When any queried data source returns no series or numbers, the expression engine returns `NoData`. For example, if a request contains two data source queries that are merged by an expression, if `NoData` is returned by at least one of the data source queries, then the returned result for the entire query is `NoData`.
+For more information about how [Grafana Alerting]({{< relref "../../../alerting/" >}}) processes `NoData` results, refer to [No data and error handling]({{< relref "../../../alerting/alerting-rules/create-grafana-managed-rule/#no-data-and-error-handling" >}}).